Many water filtration devices utilize granular water filtration media contained in a pressure vessel through which unfiltered water is treated. As shown in FIGS. 1a, 1b, 2 and 3, such conventional water filtration devices include an exterior pressure vessel in which a water distributor is positioned near the top of the pressure vessel. Water to be filtered is introduced into the vessel through the water distributor. The water to be filtered then flows through granular media resting in the bottom of the pressure vessel. Initially, the granular media presents a generally flat, circular upper surface to the unfiltered water. An underdrain is positioned at the bottom of the pressure vessel to collect the filtered water after it passes through the granular media and suspended and dissolved material in the water collects on the granular media. The water flow during filtration is typically downward, as in shown in FIG. 1a.
It is desirable for such water filtration devices to achieve fast flow rates while maintaining a high capacity for removing contaminants. Flow rates are dependent, in part, on the exterior surface area of the granular media exposed to the flow of unfiltered water and the length of the flow path. The length of the flow path is typically the depth of the granular media, commonly called the bed depth. Unfortunately, both the exterior surface area of the granular media and the flow path length are relatively fixed in conventional filtration devices like that shown in FIG. 1a.
Moreover, the flow path in many conventional water filtration devices is not widely distributed throughout the filter media, but rather quickly forms into one or a few arbitrary flow paths during the filtration process. When arbitrary flow paths form, the filter media adjacent the flow path becomes overused and prematurely exhausted, with the rest of the filter media underused. One such arbitrary flow path is shown in FIG. 3, where the path of least resistance to the water is a shortened path directly under the water distributor to the underdrain.
To compensate for the anticipated inefficient use of only a part of a media bed resulting from formation of arbitrary flow paths, the bed depth and volume is increased. Extra media is thus required, increasing the overall weight of the filtration device undesireably.
Another inherent problem with many conventional water filtration systems is the movement of the granular media during filtration. Most granular media work best if their individual grains are in a static state and are not allowed to move about during the normal filtration processes. In an effort to obtain high through put, however, individual grains of the granular media are often disturbed. Buildup of contaminants on the exterior surface of the media bed can also disturb the individual grains when high flow rates are used, as is shown in FIG. 2 where a channel along the vessel wall is temporarily dug and a mound is temporarily formed on the surface.
Conventional water filtration systems employing granular media typically require some type of media recharging after a period of use because the granular media gradually becomes covered with the waste material and efficiency is reduced. To periodically regenerate the granular media, a backwash cycle is invoked. As is shown in FIG. 1b, the normally downward flow from the distributor to the undedrain is temporarily reversed. In the backwash cycle, water is transmitted through the underdrain, up through the granular media, and out the water distributor, carrying with it the waste materials. During this process, the granular media becomes fluidized.
Naturally, an efficient backwash cyle is desireable. However, with conventional systems, when then backwash rate is speeded up to maximize backwash rates and minimize filter downtime, excessive flow rates can cause the granular media to flow out of the filter out the water distributor. Filter media which is lost must then be replaced. In addition, backwashing of conventional systems often takes an excessive amount of time, typically 5-30 minutes, when water filtration operations must be suspended.
It is against this background that the significant improvements and advancements of the present invention have taken place in the field of granular media water filtration.